1. Field of the Invention
This invention relates to optical systems having a function of correcting the shake of an image at the focal plane against vibrations of the housing thereof, or so-called "image stabilizing" function and, more particularly to optical systems having the image stabilizing function in which a movable lens unit for decentering is moved in directions, for example, perpendicular to an optical axis to effect stabilizing of the image, while still permitting the optical performance to be prevented from being lowered particularly when in close-up photography.
2. Description of the Related Art
In the use of a camera on a running car, flying aircraft, or other moving vehicles, vibrations are propagated to the photographic system (taking lens), causing an image to shake at the focal plane. In particular, for a photographic system of long focal length to be used, when the camera is held by hand, it becomes difficult to suppress the angle deviation of the photographic system from a desired line of sight. As the housing of the photographic system tilts accidentally, displacement of the image on the focal plane occurs to a magnitude depending on the angle of inclination of the housing with respect to the line of sight and the focal length of the photographic system. For this reason, the camera for still pictures gets a problem that the exposure time must be short enough to avoid deterioration of the image quality. The motion picture camera, too, suffers a problem that it becomes difficult to keep the composition for pictures to a desired setting. On occasions such as these, therefore, it becomes necessary to compensate the photographic system against small accidental angle deviations thereof from the line of sight so that displacement of the image at the focal plane, or so-called shake of the image, does not take place.
There have been previous proposals for providing a stationary image in optical systems as, for example, disclosed in Japanese Laid-Open Patent Application No. Sho 50-80147, Japanese Patent Publication No. Sho 56-21133 and Japanese Laid-Open Patent Application No. Sho 61-223819.
In Japanese Laid-Open Patent Application No. Sho 50-80147, two afocal zooming sections are used to form a zoom lens, wherein zooming is performed in each of these sections in such relation that, letting the angular magnifications of the first and second zooming sections be denoted by M.sub.1 and M.sub.2, respectively, M.sub.1 =1 -1/M.sub.2 is maintained, and also wherein the second zooming section is held in fixed spatial alignment with the line of sight to thereby correct the image shake. Stabilization of the image is thus achieved.
In Japanese Patent Publication No. Sho 56-21133, vibrations of the housing of the optical instrument are sensed by a detector. Depending on the output signal from the detector, an optical member constituting part of the optical system is moved in a direction to cancel the accidental displacement of the image. The image is thus maintained stationary.
In Japanese Laid-Open Patent Application No. Sho 61-223819, a variable angle prism of the refraction type is arranged in front of a photographic system to deflect light rays that are going to the image frame, wherein the apex angle of this prism is made equal to the angular deviation of the photographic system from the line of sight. The image is thus stabilized.
Besides these, in Japanese Patent Publications Nos. Sho 56-34847 and Sho 57-7414, one of the lenses of a photographic system is held in fixed spatial alignment with the line of sight so that, when the housing of the photographic system vibrates, a prism is generated to deflect the light rays that enter the film gate. The stabilized image is thus obtained at the focal plane.
It is also known to utilize an acceleration sensor for detecting the vibrations of the housing of the photographic system. The output signal of this sensor is translated to the transverse movement of one lens unit of the photographic system across the optical axis thereof. Thus, the image stabilizing function is realized.
In general, for the image stabilizing photographic systems of the type in which a lens unit is made movable for correcting the image shake, there have been great demands for their operating mechanisms to have a high dynamic range for correction of the image shake and also for the image stabilizing lens unit (movable lens unit) to have a short range of movement or rotation.
As the movable lens unit is decentered, aberrations are produced. If the decentering coma, decentering astigmatism, decentering chromatic aberrations and decentering curvature of field are large, the image is caused to blur, even though the image is stabilized. For example, if large decentering distortion is produced, the moved amount of the paraxial zone in the image frame differs from that of the marginal zone. For this reason, if the movable lens is transversed to a distance determined to correct the image shake in the paraxial zone, it results in the marginal zone that a similar phenomenon to the image shake takes place, causing the optical characteristics be lowered extremely.
To provide the optical system with the image stabilizing function, therefore, it is demanded that, when the movable lens unit is decentered either by moving in the direction perpendicular to the optical axis, or by rotating about a point on the optical axis to small angles, the amount of decentering aberrations produced is limited to a minimum in order to minimize the detraction from the professional image quality. For another purpose of minimizing the size of the housing of the instrument containing the optical system, it is also demanded that larger shaking movements of the image can be corrected by smaller transverse or rotative movements of the image stabilizing lens unit, in other words, the so-called decenter sensitivity (the ratio of the corrected amount of shaking movement, .DELTA.x, of the image to the unity of decentering movement, .DELTA.H, or .DELTA.x/.DELTA.H) is large enough.
Of the types of image stabilizing optical systems, the one having the additional optical member arranged to be spatially fixed against vibrations, is not suited to be used in the instrument whose prerequisites in design are partly in small size and light weight, because difficult techniques must be used to support this optical member and also because the optical system is hardly constructed in compact form. Another type which uses the variable angle prism in front of the photographic system has an advantage that, when correcting the image shake, almost no decentering aberrations except decentering chromatic aberrations are produced, but there are drawbacks that the drive member is necessarily large in size and that the decentering chromatic aberrations produced by the prism are difficult to correct by any simple techniques. The other type which decenters part of the optical system, or one lens unit, is amenable to the techniques of minimizing the size of the instrument by making appropriate the selection and arrangement of the lens unit for decentering, but suffers a difficult problem of realizing sufficient correction of large displacements of the image by sufficiently short decentering movements while properly correcting the various aberrations produced by the decentering, i.e., the decentering coma, the decentering astigmatism, the decentering curvature of field, etc.
As another related proposal, there are U.S. Pat. Nos. 4,974,950 and 5,000,549.